This invention relates to an air conditioning control system for automotive vehicles, which has a function of automatic demisting control responsive to humidity on the inner surface of a windowpane of a vehicle compartment.
an air conditioning control system of this type has been proposed by Japanese Provisional Patent Publication (Kokai) No. 63-25158, in which an actual value of humidity on the inner surface of a windowpane of a vehicle compartment is compared with a predetermined value which is assumed when the inner surface of the windowpane is in a state immediately before a mist occurs thereon, the temperature of the inner surface of the windowpane is sensed, and the predetermined value is varied depending upon the sensed temperature, thereby enabling to detect the state immediately before a mist occurs even when the temperature of the inner surface of the windowpane is below 0.degree. C.
However, according to the above proposed conventional control system demisting control is started when it is determined that the sensed actual humidity on the inner surface of the windowpane becomes higher than the predetermined value, while the demisting control is stopped when it is determined that the sensed actual humidity becomes lower than the predetermined value. Therefore, there can occur hunting or frequent alternate starting and stopping of the demisting control, which can cause failure in the air conditioning system to be controlled, such as seizure between clutch plates of an electromagnetic clutch of a compressor of the air conditioning system. Further, humidity sensors employed have variations in characteristics due to machining tolerances, which adversely affects the accuracy of the demisting control.